The process known as electrophoresis involves the migration of charged molecules through a suitable retarding medium under the influence of an electric field. Generally, the compounds of higher molecular weight migrate at a slower rate through the medium than do the compounds of lower molecular weight. Devices have been provided previously for carrying out electrophoresis. An example of such a device is U.S. Pat. No. 4,415,418 in which a tray is provided with a raised platform at the center. Removable partitions are placed in the tray at opposite ends of the platform, and a conventional electrophoresis gel is poured over the platform to form a thin layer. When the gel has cooled, the partitions are removed. A comb is provided to form wells across the surface of the gel. Substances that are to be subjected to electrophoresis are delivered into each of the wells, and the tray is at least partially filled with an electrolyte buffer. Electrodes are positioned at each end of the tray and a sufficient voltage difference is applied to the electrodes to cause migration of the molecules of the substance in the wells across the length of the gel, separated according to their molecular weight. After electrophoresis, the gel is removed from the original casting tray, and placed in a dish containing depurination solution. Approximately thirty minutes later this solution is poured out by tipping the dish toward one edge while the gel is held with the fingers. It is important to use great care during this procedure to prevent the gel from breaking because there is no gel support structure and subsequent processing is possible only with an integral gel. A denaturation solution is then added to the dish and incubation is continued for approximately thirty minutes. Again, the solution is carefully poured off. Then neutralization buffer is added and incubation is continued for thirty additional minutes.
In accordance with conventional techniques, transfer of the nucleic acids is accomplished by placing a piece of filter paper, which is as wide as and longer than the gel, on a platform which is suspended above a solution of 10.times. saturated saline citrate buffer (SSC). The ends of the filter paper are long enough to hang off the ends of the platform and dip into the 10.times. SSC. Thus, the filter paper acts as a wick to absorb the SSC solution. The gel is removed from the dish and placed on top of the filter paper saturated with 10.times. SSC. Next, a piece of membrane filter paper which is the same size as the gel is saturated with 10.times. SSC and placed on top of the gel. The nucleic acids are eventually bound to the membrane filter paper. Another piece of saturated filter paper, the same size as the gel, is placed on top of the membrane. The entire layered unit is then smoothed to remove any air bubbles that may exist between the gel and the filter paper. Finally, a stack of paper towels, the same size as the gel, is positioned on top of the layered unit.
Over a period of about 12 to 16 hours, the 10.times. SSC solution is drawn up through the gel by capillary action and the nucleic acids are transferred out of the gel into the membrane above. The paper towels absorb the excess buffer and provide the force for capillary action. At the end of the transfer period, the entire layered unit is disassembled and the membrane is removed for hybridization. This technique is described in an article Southern, E., "Detection of Specific Sequences Among DNA Fragments Separated by Gel Electrophoresis," . J. Mol. Biol., 98:503 (1975).
Although the trays such as the one described in U.S. Pat. No. 4,415,418 are convenient for carrying out electrophoresis, they are not suitable for situations where a large number of samples must be tested in a relatively short period of time.
Therefore, the prior art uses a tedious multi-step, multi-apparatus process for preparing nucleic acid fragments for subsequent hybridization. Four steps are generally undertaken to achieve preparation of the sample for hybridization. Electrophoresis was previously described. Depurination removes purine bases from nucleic acids. Denaturation involves separating the strands of nucleic acids and breaks down the depurinated nucleic acids into suitable size to allow eventual transfer of the fragments out of the gel. Transfer involves allowing the fragments to go out of the gel onto the porous membrane.